1. Field of the Invention
The present invention relates generally to chemical mechanical planarization (CMP) of semiconductor devices, and more particularly, to a method and apparatus for rejuvenating a CMP chemical solution used in a first CMP process for reuse in a second CMP process.
2. Description of Related Art
CMP is a well-known process used to remove and planarize materials on a semiconductor device such as copper, tungsten, aluminum, silicon, silicon dioxide, or silicon nitride. As part of the semiconductor device fabrication process, these types of materials are normally deposited on the surface of a semiconductor device and then removed and planarized using a CMP process.
Prior art FIG. 1 is a perspective view of a CMP system used to perform a conventional CMP process. In FIG. 1, the CMP system 5 includes a polishing pad 10, placed on a rotating table 12. The semiconductor wafer 14 containing the semiconductor device is held in a rotating carrier 16, and the front surface 17 of the semiconductor device on the wafer 14 is rubbed against the polishing pad 10 to planarize the semiconductor device.
During a conventional CMP process, a chemical liquid 18 is also required and is delivered to the CMP system 5 by a first delivery device 7. Although not shown, typically a fine particle abrasive such as alumina or silica, normally already mixed into the chemical liquid 18 and known conventionally as a slurry, is also required for the CMP process. The diameter of the abrasive particles typically ranges from ten nanometers to ten microns. The abrasive particles need not be already mixed in the chemical liquid 18, but rather may be embedded in the polishing pad 10. Alternatively, the abrasive particles may also be separately delivered to the CMP system 5 by a second delivery device (not shown) and mixed with the chemical liquid 18 on the polishing pad 10. In operation, the chemical liquid 18 and/or slurry is used to continuously wet the polishing pad 10 while the pad 10 is mechanically rubbed against the front surface 17 of the semiconductor device enabling removal and planarization of the deposited material on the wafer 14.
Recently, CMP has received a tremendous and growing amount of investigation and engineering as an enabling technology for manufacturing high-speed semiconductor devices. This is because high-conductivity copper is now being used as the interconnect material (replacing aluminum) to connect multiple semiconductor devices on a semiconductor device. With the use of copper, more and more layers are possible on a single semiconductor device and in a more compact area With the additional layers, the CMP process is used more frequently since each such layer must be planarized prior to adding subsequent layers. Thus, the CMP process is becoming increasingly more necessary as more layers are formed and increasingly more important to the overall semiconductor manufacturing process.
Two areas of concern in the CMP process are the high cost of consumables used in the CMP process and the environmental impact of discarding used CMP chemical liquids. The high cost of consumables generally stems from items such as the chemical liquid or slurry and polishing pads of FIG. 1, to name a few. For example, a copper CMP process may require about 600 cubic centimeters of chemical liquid for each semiconductor wafer processed. At this rate, a semiconductor manufacturing facility that produces 5,000 completed semiconductor wafers each week, and that requires six copper CMP processes for each completed semiconductor wafer, may require about one million liters of chemical liquid each year for the copper CMP process. At current slurry costs of about $10.00 per liter, this translates to a cost of over ten million dollars annually.
Waste disposal is another concern in the CMP process. Prior art FIG. 2 is a typical disposal system for CMP waste in a semiconductor manufacturing facility. In FIG. 2, a chemical liquid or slurry 20 contained in storage tank 25 is sent to the CMP system 30, such as the CMP system 5 of prior art FIG. 1. The xe2x80x9cusedxe2x80x9d slurry or chemical liquid from the CMP process flows through a drain 31 and is sent to a facility 35 for adherence to environmental regulations.
For a copper CMP process, this facility might remove dissolved copper in order to meet Environmental Protection Agency (EPA) requirements for maximum permitted contamination or effluent levels. For example, these EPA requirements for maximum permitted effluent levels may be 0.6 parts per million. In a copper CMP process on semiconductor wafers of 200 millimeters in diameter, with a film of deposited copper that is one micrometer in thickness, the concentration of copper in the effluent produced during copper CMP processing would be about 500 parts per million. This effluent would likely require costly procedures to remove the copper contained in the effluent.
To relieve these concerns, several solutions have been suggested. For example, in U.S. Pat. Nos. 5,664,990 titled xe2x80x9cSlurry Recycling in CMP Apparatus,xe2x80x9d and 5,755,614 titled xe2x80x9cRinse Water Recycling in CMP Apparatus,xe2x80x9d a solution of capturing a used slurry (in one patent) and rinse water (in the other patent) after the CMP process to continuously blend the used slurry or rinse water with fresh slurry is disclosed. However, there is no disclosure in these patents for removing dissolved materials, such as dissolved copper, from the slurry or rinse water. Only a filtration system is disclosed that removes particles that have not been dissolved in the slurry or rinse water. Thus, the recycled slurry still contains much of the dissolved material that was removed from the semiconductor wafer during the CMP process, which, in turn, degrades the quality of the recycled slurry.
U.S. Pat. No. 5,791,970 titled xe2x80x9cSlurry Recycling System for Chemical-Mechanical Polishing Apparatusxe2x80x9d also discloses a manner of recycling CMP slurry using an endpoint-monitoring system. The endpoint-monitoring system monitors the impact of the slurry on the removal rate of the CMP process and accordingly controls the rate of recycled slurry usage, thereby reducing any negative impact the recycled slurry may have on the CMP process. This solution, however, much like the prior patents discussed above, does not provide for removing the dissolved material found in the effluent that is removed from the surface of the semiconductor wafer. Again, the recycled slurry contains contaminated, dissolved materials that degrade the quality of the recycled slurry and affects the performance of the CMP process using that recycled slurry.
A need therefore exists for a CMP process that alleviates the high costs of consumables used in the CMP process and that reduces waste in such process. Any solution to this need must be capable of doing more than merely filtering particles in the slurry. The solution must be capable of removing dissolved materials from a used slurry or chemical liquid, without using harmful chemicals that would detrimentally impact the performance of the CMP process.
The present invention provides for a method of rejuvenating a chemical solution used in a first CMP process for reuse in a second CMP process. This rejuvenation method is performed in two steps. A first step uses the chemical solution in a first CMP process to remove material from a semiconductor device undergoing the first CMP process. This step produces an effluent that contains a dissolved first species removed from the semiconductor device during the first CMP process. The second step involves treating the effluent by removing the dissolved first species to produce a rejuvenated chemical solution. The rejuvenated chemical solution may then be used in a second CMP process.
The treating step, in one embodiment of the method of the present invention, includes a step of electroplating the effluent to remove the dissolved first species. The dissolved first species may include, for example, copper, tungsten, aluminum, silicon and the like. In another embodiment of the treating step of the method of the present invention, ion exchanging is performed to remove the dissolved first species from the effluent.
In a further embodiment of the present invention, the treating step of the method of the present invention may be performed by precipitating the dissolved first species. In yet another embodiment, an additional filtering step is performed to remove particles from the effluent, which may also be removed using a centrifuging method.
In a still further embodiment of the present invention, as part of the treating step of the method of the present invention, the step of adding a second species, such as an oxidizer, to the rejuvenated chemical solution is included. This adding step, in this embodiment, may be performed after, or simultaneous to, the treating step of the method of the present invention. The adding step may also be performed using an electroplating device.
The apparatus of the present invention includes, in one embodiment, a CMP means for removing the effluent from a semiconductor device and a treating means for treating the effluent after it has been removed. The CMP means produces the effluent containing a dissolved first species removed from the semiconductor device. The treating means then treats the effluent by removing the dissolved first species to produce a rejuvenated chemical solution. The treating means includes, in several embodiments, an electroplating device and an ion exchange device. The CMP means, in one embodiment, includes a polishing pad, a rotating table, a wafer containing a semiconductor device and a rotating carrier to hold the wafer. The CMP means further includes delivery devices to deliver a chemical solution to the wafer and a waste device to remove used chemical solutions from a CMP process.
It is noted that the CMP means is not limited to the components described in this embodiment, but includes all well known components used to CMP a semiconductor device. For example, instead of a rotating table and/or a rotating carrier, said CMP means may include any mechanism that can cause the relative motion of the wafer against the polishing pad.